Well, if it's about a hosting plan, I suggest you use "unlimited*". You could make a sidenote somewhere that technical limits do apply.
When you state "1 million emails", that is a much harder claim. When someone actually tries to make 1 million mailboxes or forwards or whatever, it's very possible H-Sphere will not like it at all But when you give a hard number (even a ridiculously high one), people have a right to complain if it doesn't work